hello to all,
can you tell me what in the x86 controls the supervisor(kernel)/user mode?
thank you

Protected and real mode are controlled by bit 0 of CR0, a 1 gets you
into protected mode, a 0 into real mode.

  "A compiler that generates better code than a good
   assembly programmer? That'll be the day."
  (Michael Abrash)

This distinction is used by the page-level protection mechanism.  In
particular,  there is a specific bit in each page table and page directory
entry (the U/S bit) which controls whether (U/S=1) or not (U/S=0) this page
(directory) is accessible to User or Supervisor level processes.

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Carlos,
  you could be looking at processor protection rings - when an application
runs in a ring certain restrictions apply to it. The 386+ has 4 rings -
ring 0 (kernel) having the most access and ring 3 (user) having the least.
The idea is that the O/S sits in ring 0 and applications can't do things
like change its data or get it to call code in an applocation that can
cause it to crash because they sit in a higher ring. The rules are ....

An application running in a ring cannot access the data of any lower rings
An application in a ring cannot call code in any higher rings
Only code in ring 0 can access the hardware directly (that's why drivers
have to be ring 0)

  This is meant to make the O/S crash proof. NT only uses the 2 rings, ring
0 and ring 3.

  I can't remember how to tell which ring code will run in but I can take a
look and find out. Hope this is of some help.

Regards
-DelBoy

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So much to do - So few machine cycles
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said:

:can you tell me what in the x86 controls the supervisor(kernel)/user
:mode?

There is no facility in the x86 family which directly corresponds to the
supervisor/user state found in the IBM 370 architecture, among others. The
closest x86 capability is "protected mode", which was first introduced in
the 80286, and further enhanced in the 80386 and above. However, unlike
supervisor mode, protected mode also enables a number of other protection
mechanisms, including changes to the basic addressing scheme which are not
compatable with unprotected (aka 'real') mode.

If you have more specific questions, post here, or by email.

Best,
        -- Chuck Crayne
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